The invention relates to the generation of pulse width modulated electrical waveforms. Pulse width modulation is susceptible of many applications, in particular, in system control where the modulation is varied for AC voltage control purposes.
It is well known to use pulse width modulation control techniques in conjunction with force commutated thyristor inverters or other types of switching amplifiers, in order to efficiently produce, from a fixed magnitude direct current input voltage source, an alternating current outut voltage whose fundamental varies in amplitude at will from zero up to a higher limit imposed by the value of the direct current input voltage.
For instance, two reciprocal force commutated thyristors in an inverter power "pole", or two reciprocal transistor-diode pairs connected in the output stage of a switching amplifier, can be activated through the process of pulse width modulation so as to generate across the load waveforms of different selected patterns. (See Electronic Engineers Handbook, McGraw-Hill, 1975, pp. 15-39 to 15-42).
The difference in amplitude of the fundamental wave at the output depends on the size and number of notches present which are defined by transitions between the two constant amplitude levels of the pulse width modulated wave. Typically, a pulse width modulated waveform looks like a train of square pulses of same amplitude and different duration symmetrically disposed in time about a cross-over point recurring at a predetermined repetitive rate, and in which the notches represent the negative image of such series of pulses. Although two waveforms of different patterns may have the same rate of recurrence, i.e., define a fundamental wave of same frequency, the amplitudes of the fundamental waves will be different depending on the size and number of notches during each cycle. Thus, the number of notches per cycle of the generated pulse width modulated wave, their width and their position relative to the associated cross-over point govern the amplitude of the outputted fundamental wave. The presence of a notch is indicative of a change in the conducting pattern of the force commutated thyristors, or the transistor-diode pairs, as the case may be, which are being controllably switched in accordance with a particular pulse width modulation pattern.
As a practical application, the load may be, for instance, one of the phases of a frequency controlled induction motor. Control by pulse width modulation is effected to vary the motor terminal voltage when the frequency varies, as required to keep the flux constant. Another typical application of pulse width modulation is regulating the output voltage on the critical bus of an Uninterruptible Power Supply.
It is known from U.S. Pat. No. 3,947,736, dated Mar. 30, 1976 of Byers to digitally form notches for a PWM wave in accordance with a predetermined arithmetic function.
It is knonw from "A Digital Logic PWM Speed Control for Single and Polyphase AC Motors" by T. Masur, IEEE-IAS Conference Record of 1973 Annual Meeting, pp. 1-9, to use a Read Only Memory in a PWM control scheme to generate parallel sine-weighted pulse trains which are applied for speed control to several parallel AC induction motors.
It is known from U.S. Pat. No. 3,890,620 issued June 17, 1975 of Donald J. Toman and Roland Coulter, to store in a digital memory digital sample point values signifying various modulation levels required at successive points in time and to read out in timed sequence such values and apply them to modify a carrier wave so as to create modulation thereon.
However, the prior art techniques for modulating an output wave by digital means do not address themselves to the problem of generating a multiplicity of modulation output waves of predetermined quality and of outputting output waves selected for their predetermined characteristic as a function of time and applied in direct relation to a predetermined process variable and according to a control scheme.
It is also known to generate output waveforms having appropriate notches to yield any desired output. See in particular J. Zubek, A. Abbondanti and C. Nordby, "Pulse-Width Modulated Inverter Motor Drives With Improved Modulation" paper presented as IEEE Conference Record of Ninth Annual Meeting of IAS, pp. 998-1006, 1974. It appears from this paper that the merits of a modulation scheme depend to a great deal upon the choice of appropriate patterns of the pulse width modulated generated waves. However, conventional methods for the generation of output waveforms having the appropriate notches to yield any desired output are inherently limited because they use analog signal processing techniques, such as waveform generators, level comparators, time-ratio or transconductance multipliers or analog switches. Analog pulse width modulation methods can satisfy the requirements of actual thyristor systems only if operated within a limited range of output voltage or frequency. Beyond such range, "multi-mode" modulation is required often involving changing many times the operative mode of the modulator. This results in increased circuit complexity and causes waveform distortions which impair the commutation ability of the inverter. Multi-mode modulation may, at a certain point, become totally unacceptable. A drawback typical of this type of modulation lies in the lack of flexibility to choose a waveform having notches placed at the most desirable position within the cycle. Theoretically, for a desired output amplitude and a given set of system constraints taking into account the limitations of the inverter's commutation circuit, it is possible to analytically determine the pattern of notches which is most suitable. Actually, with an analog system such optimum pattern will rarely be obtained. Only an approximation is possible in practice, which often may be acceptable to a certain degree. However, there are instances where the range of modulation requies the output waveform to vary from 0 to 100% of the maximum amplitude assigned to the fundamental in the system. In such case, the analog modulation methods of the prior art yield at times waveforms of poor quality, far from the desired theoretical shape. Current distortions may become rather large.
The object of the present invention is to overcome the drawbacks of the analog modulation methods of the prior art, to make it possible to generate waveforms having the optimum quality defined by theoretical analysis for any assigned output level, and to implement the generation of such perfect waveforms with circuitry of acceptable complexity and cost, at any rate, with hardware of less complexity and cost than would be required with analog modulation circuits. The result is that the modulation method according to the invention allows a wider range of modulation without sacrifice of commutation capability and efficiency, even for controlled systems of high power rating.
The invention calls for digital techniques instead of analog modulation, and permits a synthesization of the desired waveform in such a way that a considerable freedom of choice is possible in obtaining the waveform pattern as never attained with the conventional analog techniques. Among all theoretically possible waveforms, the waveforms to by synthesized are preferably those which in practical situations have been proved to possess the highest quality, for instance by exhibiting the lowest content of undesirable harmonics within the constraints of a power system given for the implementation of the invention.
More specifically, the novel pulse width modulation method according to the invention is applicable for control of force commutation inverters as used with PWM to achieve output voltage control in variable frequency AC drives or in regulated AC power source such as Uninterruptible Power Supplies. The proposed modulation method is particularly advantageous for these applications since it allows reducing the harmonics to the minimum level conceivable, and offers the best possible use of the installed commutation KVA rating of the inverter system, or of the KVA handling capability of the output transitors, if a transistorized power system is used. The invention offers the best utilization of the system's power stages, thereby to reduce the cost per handled KVA of the PWM system. Moreover, the method according to the present invention affords a substantial improvement in the overall control operation by the smoothness in operation and the resolution of voltage control attained. This is a particularly attractive feature in variable frequency motor drives of large rating which, otherwise, are known to be handicapped by the slow turn-off characteristic of the thyristors. Another remarkable advantage resides in substantial savings with the hardware. Digital treatment on a PWM system suitably designed in accordance with the present invention allows to establish complex LSI logic system circuitry having elaborated circuit functions compacted in small modules and packages. This results in reduced cost and more reliability in the AC motor drives. Therefore, this type of drive can be competitive with DC motor drive despite the greater complexity of the control electronics required in AC drives. Further cost reductions are possible through customized LSI packages implementing the modulation functions in a very limited number of logic packages, and ultimately on a single semiconductor chip.